Expert Interview with Professor Markus Knupp, MD, on High-Precision Care: The Modern Ankle Prosthesis

Professor Dr. med. Markus Knupp is an internationally recognized specialist in disorders and surgery of the ankle joint and the head of “Mein Fusszentrum AG” in Basel, Switzerland. With many years of experience and several thousand successfully performed procedures, he specializes in treating degenerative diseases of the ankle, particularly ankle osteoarthritis.

His focus is on tailored, evidence-based treatment concepts that offer patients state-of-the-art therapeutic options. He employs innovative techniques such as precise 3D planning and patient-specific instruments to achieve anatomically appropriate and durable solutions for advanced osteoarthritis. As a leading foot surgeon, Prof. Dr. Knupp places great importance on a balanced decision between joint-preserving measures and surgical interventions.

Special attention is given to total ankle replacement, which he performs using the most up-to-date technical standards. In addition to implanting ankle prostheses, he also offers tried-and-true joint fusion (arthrodesis) when it is the right solution in certain cases. He is also proficient in complex conversion surgeries, for example converting a fusion to a prosthesis or vice versa, which requires specialized biomechanical expertise. Extensive reconstructive procedures on the ankle round out his field of expertise.

At his practice “Mein Fusszentrum” in Basel, Prof. Dr. Knupp works with cutting-edge medical infrastructure and an interdisciplinary team to ensure comprehensive diagnostics, precise surgical planning, and individualized follow-up care. As a professor at the University of Basel, he plays a key role in advancing ankle surgery and regularly publishes scientific studies that reflect the current state of the art. This makes him a significant voice for innovative and sustainable solutions in the treatment of ankle disorders.

The editorial team of Leading Medicine Guide spoke with Prof. Dr. Knupp to learn more.

 

Ankle replacement is an advanced treatment option for patients suffering from severe osteoarthritis or other conditions of the ankle joint. Thanks to innovative technical procedures and customized implants, it is now possible to significantly improve mobility, reduce pain, and enhance long-term quality of life. In this context, precise planning and modern surgical techniques play a decisive role in achieving optimal outcomes and enabling sustainable healing. 

An ankle prosthesis is generally indicated when the joint is so severely affected by advanced osteoarthritis, major injuries, inflammatory diseases, or structural damage that conservative treatment is no longer sufficient. Typical causes include chronic cartilage damage, persistent pain, restricted range of motion, and increasing joint instability that significantly impairs quality of life.

“The causes of ankle osteoarthritis can be divided into three groups. The largest share, about 80 percent, are so-called post-traumatic osteoarthritis. This means the condition occurs after an injury—for example, an accident with a torn ligament or an ankle fracture. These injuries are the main cause of osteoarthritis. The second group accounts for roughly ten percent of cases and includes patients with an underlying disease, such as polyarthritis (a rheumatologic condition) or hemochromatosis, meaning blood or coagulation disorders.

Conditions like psoriasis can also cause osteoarthritis. This group mainly includes chronic inflammatory or systemic diseases. The third group is referred to as primary osteoarthritis. These are cases where medicine can hardly identify a clear cause. These patients have neither a known underlying condition nor a prior injury. Genetic factors or other unrecognized causes may lead to osteoarthritis. Although not everything is understood yet, in these cases the condition seems to arise without a clear trigger”, explains Prof. Dr. Knupp.

The diagnosis is made through a thorough clinical examination in which the physician assesses joint mobility, tender points, and stability. Imaging is also used—primarily X-rays—to visualize cartilage damage, bony changes, or deformities. If necessary, MRI or CT scans can be added to better evaluate the soft tissues and the exact extent of the damage. 

“When a patient presents to us, the diagnostic work-up begins with a detailed medical history. We record how the symptoms started, how long they have been present, and how severe the impairment is. We ask about limitations in daily life, leisure, and sports caused by pain. It is also particularly important whether the symptoms affect professional life or restrict the ability to work.

We also ask whether certain shoes can no longer be worn—in Switzerland, patients often report that most sports are hardly possible anymore, but skiing still is because they are in rigid ski boots. Such clues help us better understand the extent of the load. Furthermore, it is relevant whether there were past accidents, injuries, or surgeries that could influence the symptoms. We clarify whether there were prior infections, ligament surgeries, or wound-healing problems. We also ask about health factors from childhood, such as malalignment, flat feet, or high arches. In addition, we are interested in which measures the patient has already tried—whether medications, injections, ointments, physiotherapy, or other methods—and whether these were helpful.

In the clinical examination, we look closely at the foot: Is the skin intact? This is highly relevant when planning implantation of an ankle prosthesis, as the skin must be closed again. We also check for signs of infection, especially in patients who had a procedure less than two to three years ago, as there is a risk that bacteria remain in the tissue. Joint stability is another focus. Is the joint stable—are the ligaments intact?

If the ligaments are weak or damaged, they may need to be corrected during surgery. We test the ligaments and tendons—how do they feel? We also determine the limb axis: Is it straight or misaligned? Malalignment can lead to unilateral wear, similar to a tire mounted crookedly, which affects the longevity of the prosthesis. We can also see this on the shoes: if one side is more worn, it indicates axis malalignment”, explains Prof. Dr. Knupp, before turning to the necessary imaging: 

“Imaging is, of course, part of the diagnostics. We take weight-bearing X-rays because the foot looks much more realistic in this position than when lying or sitting. This is sometimes difficult for patients to understand, as they often have already had X-rays at their primary care provider, but weight-bearing images are necessary to capture the actual conditions. We also obtain a full-length leg image to rule out malalignment of the knee or hip and to ensure the leg is in the ideal axis. If abnormalities such as cysts or other anomalies are visible in the joint, we supplement diagnostics with MRI or CT. CT, in particular, has become very important for us, especially in complex cases, as it enables 3D planning. For this, we need a CT of the foot, the upper and lower ankle joints, and the knee. Based on these images, we can plan patient-specifically and optimally prepare the surgery”.

For ankle replacement to be successful, several prerequisites and criteria must be met to ensure optimal function and long service life. The bones around the ankle joint must be stable and strong enough to allow secure fixation of the implants. This means there must be no significant bone loss or osteoporosis; otherwise, there is a risk of loosening or inadequate anchoring of the implant. 

Prof. Dr. Knupp comments: “When we have patients for whom we cannot implant a prosthesis, there are usually specific reasons. In particular, if a ligament reconstruction is also required but we recognize that it will not lead to success, we sometimes consider joint fusion as an option. These cases are characterized by very poor bone quality. You often see bone with large defects. There are two possibilities here: either we attempt to reconstruct the bone in two stages, for example with a bone graft, or we opt directly for fusion.

Another reason not to implant a prosthesis is an existing infection. In such cases, it is possible to treat the infection first and then still proceed with ankle replacement. Nerve function is also an important factor. Patients with impaired nerve function—especially diabetics with neuropathy—often have reduced sensation in the foot. If foot sensation is significantly limited, we are more cautious and more likely to choose fusion instead of a prosthesis to avoid complications. With technological advances, particularly 3D planning, the surgical procedure has become much more individualized. In the past, prostheses were generally off-the-shelf—standard models assembled from fixed components. Today, there are still various prosthesis models made up of numerous modules that can be combined individually—certainly over 30 variants.

However, the greatest challenge in ankle prosthetics lies in the exact accuracy of implantation because the joint is very small. Even a few degrees of deviation can impair function. Here, 3D planning helps, as computer-assisted methods allow more precise surgical technique and implantation. Although prostheses are still standardized, individualized planning with 3D technology enables a much better fit and greater accuracy during surgery. For the ankle, fully custom prostheses play a subordinate role, as conventional models already work very well for this application”.

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The patient’s general health plays an important role. The body must be able to tolerate the surgery well and handle the subsequent rehabilitation phase. This is especially relevant for older or multimorbid patients with underlying conditions such as heart, lung, or other chronic diseases.

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Robotics for the ankle is currently of no real interest because it simply does not provide a reliable solution in this area. That may seem unusual at first glance, since the ankle is a very small and delicate joint where one might think a robot’s precision would be advantageous.

“The problem lies precisely here: Robot-assisted surgery requires reference points to determine exact spatial position. In the knee or hip, large pins can be placed as fixed reference points and 3D sensors can capture positions. In the foot, it is completely different. There are 26 small bones that slide against and move relative to each other. There are no reliable reference points because the bones constantly shift in relation to one another.

The only reliable reference would be the tibia or possibly the fibula, but the bones below move in all directions, making precise control impossible. Therefore, the use of robotics for the ankle is currently not feasible and likely will not be developed in the coming years. The complexity is too great, and the industry is not moving in this direction at the moment because there is simply no practical solution”, says Prof. Dr. Knupp, before describing recent advances: 

“The development of ankle prostheses has changed significantly in recent years, especially since around 2018. About ten years ago, when the first generation of these prostheses was implanted, the results were mixed. Many patients—about a third—still reported considerable symptoms after surgery. Today, with the new implants, the results are much better. Patient satisfaction and prosthesis longevity have become comparable to knee replacements and in some cases have even slightly surpassed them.

This is primarily due to improvements in implantation accuracy. The enhanced technique has made it possible to position the prosthesis more precisely, which has greatly improved outcomes. The materials have also been optimized, but the key progress lies in improved surgical technique and a better understanding of comorbidities. The ankle is a complex ‘sandwich’ between the ground, the subtalar joint, and the knee above—each must be considered in terms of function and biomechanics to ensure the prosthesis works optimally. Increased knowledge of biomechanics and the interplay among these joints has contributed to significantly better results today”.

Various risks and complications can occur with ankle replacement, though these can be minimized through careful planning and professional execution. 

“The main risk in ankle surgery is infection. This is a bigger issue than in knee or hip surgery because the soft tissue envelope is much thinner, and the foot is often already compromised by previous injuries or operations. Many patients have a history of trauma or procedures, which further weakens the skin and increases risk.

Over the last ten years, however, we have learned a great deal about infection prevention. While in 2012 nearly five percent of patients developed an infection after ankle replacement, this number has now dropped to below one percent. That is comparable to the standard risks in knee or hip procedures. By adapting surgical techniques and better understanding how infections occur, we have significantly reduced these risks. There are now hardly any unusual complications—rather the classic, expected risks associated with such procedures”, emphasizes Prof. Dr. Knupp.

Postoperative follow-up after ankle replacement is crucial to promote optimal healing and ensure the long-term stability of the implant. Immediately after surgery, close monitoring follows to check the wound, minimize infection risks, and provide the best possible pain management. 

“Follow-up after ankle replacement is quite long-term. Patients must wear a special orthosis—a so-called boot—for the first six weeks after surgery. They may bear full weight and even walk without crutches as soon as they can tolerate it. Wearing the orthosis is a major advantage compared to fusion or a classic cast, as aftercare is significantly simpler. Nevertheless, caution is required: During the first six weeks, patients should avoid malloading or excessive ligament stretching.

There were considerations to shorten the duration to four weeks, but the outcomes did not improve. When the orthosis is removed after about six weeks, care usually continues. Patients are re-examined after one year, and we generally maintain contact, even if ongoing treatment is managed by local orthopedic surgeons. They send us the X-rays, and I often call patients myself to check that everything is fine and how they are feeling. We remain on standby should any problems arise; in such cases, local physicians obtain new X-rays and send them to us. For patients who live farther away, we usually see them after six to twelve months, and we also recommend follow-ups at five and ten years to monitor progress”, says Prof. Dr. Knupp.

Earlier prosthesis models had more limited longevity. It was estimated that after about ten years, roughly 80 percent of prostheses still functioned more or less well, while about one fifth had to be removed. Newer prostheses, by contrast, are designed to last around 15 to 20 years. 

Prof. Dr. Knupp explains: “I personally know patients who have had their prosthesis for more than 20 years without problems. This is comparable to knee and hip outcomes, although hips and knees generally have even better long-term results. Hip replacement is among the most successful surgical procedures overall. Ankle replacements yield good results, but they are not quite as reliable as hips or knees. If symptoms recur after twelve years, for example due to loosening or other issues, a patient can certainly expect a revision.

Fortunately, we now have the option of using so-called revision prostheses. We remove the old prosthesis and implant a new one—sometimes combined with fusion if no other solution is feasible. Ideally, however, a revision prosthesis is implanted, which—thanks to modern technology, especially 3D planning—can be performed very precisely and efficiently. 3D planning greatly simplifies assessment, shortens operating time, and leads to better results. It is also possible to ‘reactivate’ a joint that was fused in the past. Some patients had severe injuries 20 years ago, leading to fusion, and today wish to undo the fusion and receive a prosthesis. This is also feasible nowadays”, he adds, before outlining activity levels after surgery: 

“In principle, a patient with an ankle prosthesis can resume sports, though certain activities are more advisable. With respect to joint load, we recommend doubles in sports like tennis—activities with shorter distances. Experience shows that many patients—especially when pain-free—do not always strictly follow these recommendations and engage in other sports. Tennis is generally possible, but we advise against it because the forces on the joint are too great.

Skiing is fine, but snowboarding is rather not recommended due to the higher joint load. Hiking is usually no problem. Long-distance, high-intensity running, such as marathons, is hardly realistic and generally not recommended. Light interval sessions or moderate running activities, however, are reasonable. As for timing: After about three to four months, longer hikes and similar moderate activities are possible again. The final result—what the patient can achieve after the healing phase and rehab—can usually be assessed after one year, sometimes only after one and a half years. The actual rehabilitation period is quite long—most patients need between three and six months to resume most functions”. 

Thank you very much, Professor Dr. Knupp, for the detailed insights!